and the Periodic Table Chapter 18

Section 1: Structure of the Atom Scientific Shorthand –Chemical symbols consist of one capital letter or a capital letter plus one or two small letters. –1 st letter: always capitalized –2 nd or 3 rd letter: not capitalized

atom—smallest piece of matter that still retains the property of the element Atomic Components –nucleus: small, positively charged center of the atom that houses the protons and neutrons –The nucleus is surrounded by a negatively charged cloud containing the electrons called the electron cloud.

Atomic Components—continued –proton: located in nucleus; electrical charge of +1 –neutron: located in nucleus; no electrical charge –electron: move around the nucleus in the electron cloud; electrical charge of –1

The Electron Cloud Model Each electron in the cloud is much smaller than a proton or neutron. Because an electron’s mass is so small and the electron is moving so fast, it is impossible to describe its exact location in an atom.

Section 2: Masses of Atoms Almost all of the mass of an atom is in the nucleus, because protons and neutrons are so much more massive than electrons. The mass of an electron is so small that it is not even considered when finding the mass of an atom. The unit of measurement used for atomic particles is the atomic mass unit (amu). The mass of one proton or one neutron is about 1 amu.

Protons Identify an Element Atoms of different elements are different because they have different numbers of protons. The atomic number is equal to the number of protons in an atom's nucleus. The atomic number determines which element an atom is. For example, any atom that contains exactly 47 protons in its nucleus is an atom of silver.

Mass Number The sum of the number of protons and the number of neutrons in the nucleus of the atom is the mass number. If you know the mass number and the number of protons (atomic number), you can calculate the number of neutrons.

Isotopes isotope—atoms of the same element with different numbers of neutrons An isotope is written with the element name or symbol first, followed by the mass number.  Carbon-12  Boron-10  Hydrogen-3

Average Atomic Mass The mass of an element listed on the periodic table is the weighted average of all the masses of the isotopes of that element—called average atomic mass.

Calculating Average Atomic Mass Naturally occurring atoms of boron have mass numbers of 10 or 11. –About 19% of the boron in the universe is boron-10. –About 81% of the boron in the universe is boron-11. (10 x.19) + (11 x.81) =10.81 amu

Section 3: The Periodic Table Dmitri Mendelev, a Russian chemist, searched for a way in the late 1800s to organize the elements. In the modern periodic table, the elements are arranged by increasing atomic number and by changes in physical and chemical properties.

groups (families)—vertical columns in the periodic table –The groups are labeled 1-18 across the top. periods—horizontal rows of the periodic table –The periods are labeled 1-7 down the side.

Electron Cloud Structures The electron cloud is separated by energy levels. Each energy level can only hold so many electrons. The electrons in the outermost energy level are called valence electrons. ENERGY LEVEL MAX # OF ELECTRONS 1 st 2 2 nd 8 3 rd 18 4 th 32

Groups and Valence Electrons –The groups are numbered 1-18 at the top of the table. –Elements in each group have similar properties. –Elements in the same group have the same number of electrons in their outer energy level. –Except for groups 3-12, the last number of the group number tells you how many electrons are in the outer energy level.

Electron Dot Diagrams An electron dot diagram uses the symbol of the element and dots to represent the electrons in the outer (valence) energy level.

Regions on the Periodic Table

All of the elements on the left side of the table are metals. –examples: iron, zinc, copper –Most exist as solids at room temperature. –properties: shiny, conduct heat and electric current, ductile, malleable –tend to lose electrons in reactions –tend to have 3 or fewer electrons in outer energy level

All of the elements on the right side of table are nonmetals. –examples: oxygen, helium, fluorine –Most exist as gases at room temperature. –properties: poor conductors of heat and electricity –tend to gain electrons when they react with metals –Tend to have five or more electrons in outer energy level

All of the elements on the staircase section between the metals and nonmetals are metalloids. have properties of both metals and nonmetals boron—Bantimony—Sb silicon—Sitellurium—Te germanium—Gepolonium—Po arsenic—Asastatine—At

Most Active Metal—francium Nonmetal—fluorine